Forestry
An International Journal of Forest Research
Forestry 2015; 88, 345 – 358, doi:10.1093/forestry/cpv005
Advance Access publication 12 March 2015
Evidence against planting lodgepole pine monocultures in the
cedar–hemlock forests of southeastern British Columbia
W. Jean Roach1*, Suzanne W. Simard2 and Donald L. Sachs3
1
Skyline Forestry Consultants Ltd., 843 Arlington Court, Kamloops, BC, Canada V2B 8T5
Department of Forest and Conservation Sciences, University of British Columbia, 2424 Main Mall, Vancouver, BC,
Canada V6T 1Z4
3
Forest Research Consultant, 111 View Street, Nelson, BC, Canada V1L 1V8
2
*Corresponding author. E-mail: [email protected]
Received 7 April 2014
Single-species planting of lodgepole pine (Pinus contorta var. latifolia) following clear-cut logging or wildfire has
been common throughout interior British Columbia, Canada, but health problems with the species have been documented as it grows beyond the juvenile stage. We examined damage and stocking in twenty-seven 15- to 30-yearold lodgepole pine plantations that were previously declared free growing in the highly productive cedar –hemlock
forests in southeastern British Columbia, where lodgepole pine is absent from many primary forests. In order to be
free growing, stands must meet minimum tree density, height, damage and brush competition criteria as legislated by the Provincial government. Overall, 44 per cent of lodgepole pine trees had unacceptable damage
(causing them to be rejected as crop trees), and as a direct result, one-third of the plantations were no longer
defined as free growing because there were insufficient crop trees remaining. Natural regeneration of other tree
species partially compensated for the unhealthy pine. Logistic regression and odds ratio analysis associated increasing risk of damage from western gall rust with increasing soil moisture, more northerly aspects and mechanical site preparation, and decreasing risk with pre-commercial thinning treatment. Risk of damage from snow and
ice was associated with increasing mean annual precipitation, decreasing longitude and broadcast burning. Risk of
bear damage was associated with increasing soil moisture, pre-commercial thinning treatment and broadcast
burning. Based on our results, we recommend that single-species planting of lodgepole pine be curtailed in the Interior Cedar –Hemlock zone in southeastern British Columbia.
Introduction
There is a long history of introducing tree species from areas where
they are native to other areas in the world, driven by economic, aesthetic, scientific and other reasons. Lodgepole pine is one of several
North American tree species that has been planted outside of its
native range, including New Zealand and parts of Europe such as
Sweden (Gundale et al., 2014). In Sweden, lodgepole pine has
been planted on a large scale since the 1970s to meet a predicted
shortage of harvestable softwood (Engelmark et al., 2001). It is a
desirable species because it can produce 36 per cent greater
total volume growth than the native Scots pine (Pinus sylvestris
L.), regardless of site index (Elfving et al., 2011). The success of
lodgepole pine in Sweden has been associated with a more favourable soil biotic community than in native Canada (Gundale et al.,
2014). However, it is well known that transferring species to
areas outside of their natural range can also have potentially
serious consequences. For example, the introduction of lodgepole
pine to New Zealand has resulted in prolific unwanted natural regeneration, which is a threat to indigenous flora and fauna as
well as visual and landscape values, and it is now largely considered
a weed species and seldom planted (Ledgard, 2001). In Sweden,
introduced lodgepole pine has also spread outside of areas
where it was initially planted (Engelmark et al., 2001). Pests and
pathogens from the introduced lodgepole pine could potentially
spread to native tree species such as Scots pine, which have not
co-evolved with these exotic pests.
While lodgepole pine has been introduced to countries far
from its natural range, it has also been transferred outside its
productive range within British Columbia, Canada. This includes
areas where it is not prevalent naturally in primary forests, such
as the cedar –hemlock forests in the interior of the province. In
the Interior Cedar –Hemlock (ICH) zone in the Kootenay–Boundary
Forest Region in southeastern British Columbia, lodgepole pine was
absent or a minor pre-harvest species on 53 per cent of sites where
it was planted in the last 15 –30 years (British Columbia Ministry of
Forests, Lands and Natural Resource Operations, 2011). Western
redcedar (Thuja plicata Donn ex D. Don) and western hemlock
(Tsuga heterophylla (Raf.) Sarg.) dominate late successional ICH
forests, and mixtures of interior Douglas-fir (Pseudotsuga menziesii
var. glauca (Beissn.) Franco), western white pine (Pinus monticola
Douglas ex D. Don), western larch (Larix occidentalis Nutt.) and
lodgepole pine are the major species in early successional ICH
forests. The ICH zone contains the most productive forests in
# Institute of Chartered Foresters, 2015. All rights reserved. For Permissions, please e-mail: [email protected].
345
Forestry
interior British Columbia (Meidinger and Pojar, 1991). To take advantage of the high potential timber yields in the ICH zone, it is important to regenerate sites with tree species that will be healthy
and productive until they reach maturity. Lodgepole pine is
among the fastest growing species in the ICH zone for the first 10
to 30 years, which has contributed to its use as a preferred regeneration species in this zone, but by age 80, it has lower yield than
almost all other natural associated species (Vyse et al., 2013).
The long-term health of stands is largely unknown because they
are not usually monitored after they reach ‘free growing’, the
point where licensees are relieved from the reforestation obligations set by the provincial government. A free-growing stand is
comprised of trees of preferred or acceptable species that are ecologically suited to the site and meet established criteria for health,
height, density, spacing and overtopping vegetation (British Columbia Ministry of Forests, 2000) (see Table 1 for definitions of
terms). Once a stand reaches this important administrative milestone, which is usually at age 7 –20 years, the stand is assumed
to remain healthy and productive until the next harvest. There is
mounting evidence, however, that in British Columbia lodgepole
pine plantation health beyond the free-growing assessment age
is often poor (Heineman et al., 2010). Poor pine health has been
documented across six biogeoclimatic zones in southern interior
British Columbia, but health concerns were highest in the ICH
zone where 70 per cent of lodgepole pine stands previously
declared free growing no longer met this requirement by age
15 –30 (Mather et al., 2010). Two-thirds of the lodgepole pine in
the plantations studied was damaged, and 93 per cent of this
damage was ‘unacceptable’ (preventing free growing, that is,
causing the tree to be rejected as a potential crop tree).
The health of lodgepole pine plantations may decline even
further as climate change progresses (Dale et al., 2001). Climate
change is likely to have a profound and long-term impact on
forest ecosystems in British Columbia and may provoke losses of
other goods and services that sustain communities and the provincial economy. Changes in precipitation and temperature can affect
forests by altering the frequency, intensity, duration, extent and
timing of fire, drought, insect and pathogen outbreaks, windstorms, landslides, ice storms or hurricanes (Dale et al., 2001).
Forests dominated by a single species such as lodgepole pine are
expected to be particularly susceptible to catastrophic losses, as
was seen with the recent mountain pine beetle (Dendroctonus ponderosae Hopkins) epidemic in interior British Columbia (Kurz et al.,
2008). The current widespread distribution of lodgepole pine in
western North America is predicted to be dramatically altered
under a changing climate, with other species progressively favoured
and lodgepole pine potentially disappearing from much of its
current range by 2070 (Coops and Waring, 2010). Other important
conifer species in British Columbia are also expected to significantly
decrease in frequency and/or lose a large portion of their suitable
Table 1 Glossary of terms
Term
Meaning
a
Silviculture prescription (SP)
Target stocking standard (TSS)b
Minimum stocking standard (MSS)b
Well-spacedb
Free growingb
Preferred and acceptable speciesb
Unacceptable damage
Monoculture plantation
Monoculture stand
a
A silviculture prescription outlines the required management objectives, standards and timelines that the
owner of an opening must achieve, including reaching a free-growing stand.
Target stocking is the number of well-spaced preferred and acceptable trees per hectare that will, under
normal circumstances, produce an optimum free-growing crop. On circum-mesic sites, TSS in the ICH zone
has averaged 1200 stems ha21, although planting density is often higher (up to 1600 stems ha21). The TSS
is 50 to 70% higher than the MSS. Significant volume reductions are projected if stands are managed to
minimum rather than target stocking standards.
Minimum stocking standard is the lowest number of acceptable well-spaced stems per hectare required to
consider an area satisfactorily stocked at the free-growing stage.
Well-spaced trees are healthy trees of preferred or acceptable species that are at least the minimum
horizontal inter-tree distance from other well-spaced trees. The inter-tree distance is specified in the
silviculture prescription and is usually 2 m.
A free-growing tree must be well-spaced, free from damage as defined in the free-growing damage criteria,
the required minimum height specified in the SP or in the ‘Establishment to Free Growing Guidebook’, and
free from unacceptable brush and broadleaf tree competition as described in the Establishment to Free
Growing Guidebook.
Preferred and acceptable species are those species that are ecologically suited to the site. Preferred species
are those that best meet the management objectives for a site and will produce the greatest volume of
high-quality saw logs over the rotation. Acceptable species may occur as a component of mixed stands but
are not considered to be the target species for a site.
Unacceptable damage is the term we use in this article to refer to damage that prevents a tree from being
well-spaced and free growing.
Monoculture plantations is the term we use when a single species is planted on a site. Natural regeneration
may also occur, so that the resulting stands are not ‘monoculture stands’.
Monoculture stands are stands in which ≥80% of the trees on the site are comprised of one species.
British Columbia Ministry of Forests 2002.
British Columbia Ministry of Forests 2000.
b
346
Evidence against planting lodgepole pine monocultures
habitat, including subalpine fir (Abies lasiocarpa (Hook.) Nutt.), white
spruce (Picea glauca (Moench) Voss), Engelmann spruce (Picea
engelmannii Parry ex. Engelm.) and black spruce (Picea mariana
(Mill.) B.S.P.) (Hamann and Wang, 2006).
Our study investigated the current state of monoculture lodgepole pine plantations, and logistic regression and odds ratios were
used to associate certain factors with risk of damage to lodgepole
pine. In this article, ‘monoculture lodgepole pine plantations’ are
defined as those where lodgepole pine was the only species
planted; however, these stands may include other naturally occurring species. We define ‘monoculture forests’ as stands comprised
of ≥80 per cent one tree species, which may or may not be of
natural origin. The overall objective was to investigate stocking
and damage incidence in 15- to 30-year-old lodgepole pine plantations that were previously declared free growing. The study area
was the ICH forests of the Columbia Basin in southeastern British
Columbia, Canada. Previous studies of lodgepole pine health in
this area are lacking. Six questions were posed: (1) Are lodgepole
pine plantations that were previously declared free growing continuing to meet stocking and free-growing requirements past the
juvenile stage (at age 15 –30 years)? (2) How much and what
species of natural regeneration occurs in these plantations and
to what degree is it contributing to free-growing success? (3)
What are the biotic and abiotic causes of damage leading to reductions in stocking, and what are their extent and incidence levels? (4)
Are certain climatic, site or stand factors or silviculture treatments
associated with increased risk of damage to lodgepole pine from
the most prevalent damaging agents? (5) How can management
practices be adjusted to address increased risk of damage associated with these factors? (6) Given climate change predictions, is
damage from the major agents likely to worsen over time?
Methods
Study area
A total of 27 plantations were sampled throughout the ICH zone in the Columbia Basin in southeastern British Columbia, between 498 04′ N and 518
53′ N latitude and 1148 58′ W and 1188 05′ W longitude. The Columbia
Basin drains the Columbia River and covers 671 000 square kilometres,
making it the sixth largest river basin in North America. About 15 per cent
of the Basin lies within Canada (Columbia Basin Trust, 2008). In British Columbia, it extends from the Canada– US boundary north to the Kinbasket
Reservoir, and from the British Columbia– Alberta border to just west of
the Upper and Lower Arrow Reservoirs. It is located within the Kootenay–
Boundary Forest Region. Within the Basin, the ICH zone occupies valley
bottom to mid-slope positions (400– 1550 m), below the Engelmann
spruce-subalpine fir (ESSF) zone. The ICH zone has an interior, continental
climate with warm dry summers and cool wet winters and is among
the wettest zones in the interior of British Columbia (Meidinger and
Pojar, 1991). Long-term mean annual precipitation (MAP) averages
640–1128 mm y21, and mean summer precipitation (MSP) averages
211–385 mm y21. It is the most productive forested zone in interior
British Columbia (mean annual increment up to 30 m3 ha21 y21) and has
the highest natural tree species diversity of that region (site richness up to
15 species ha21) (Simard and Vyse, 1992). In the wetter parts of the ICH
zone in the Columbia Basin, wildfires have been infrequent and large
areas are dominated by climax stands, comprised of western redcedar
and western hemlock throughout, and with hybrid Engelmann-white
spruce and subalpine fir also common at higher elevations and where
cold air drainage occurs. In drier ICH subzones in the Columbia Basin, wildfires have been more frequent, resulting in seral stands that include
lodgepole pine, western larch, interior Douglas-fir, western white pine,
trembling aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera
Marsh.) and black cottonwood (Populus balsamifera ssp. trichocarpa (T. & G.)
Brayshaw). Seven ICH subzones occur in the Columbia Basin, with the most
widespread the ICHmw (Moist Warm), where the greatest number of the
sample sites were located (17 of 27). Sampling was also conducted in the
ICHmk (Moist Cool), ICHdm (Dry Mild) and ICHdw (Dry Warm) subzones.
Plantations were not assessed in the ICHwk (Wet Cool) or ICHvk (Very Wet
Cool) subzones because accessible surviving plantations of the appropriate
age could not be found, nor in the ICHxw (Very Dry Warm) subzone because
of its limited occurrence in the Columbia Basin, and British Columbia as a
whole. Elevation of the sites ranged from 680 to 1550 m, and the absolute
soil moisture regime varied from moderately dry to fresh, which was classified using methods described in British Columbia Ministry of Forests and
Range and British Columbia Ministry of Environment (2010). Slope positions
were lower, mid and upper, aspect was variable and slope gradient ranged
from 0 to 60 per cent. Characteristics of the study sites are summarized
in Table 2.
Site selection
Twenty-seven sites were randomly selected from the population of 1386
plantations in the British Columbia government RESULTS database of silviculture activities (British Columbia Ministry of Forests, Lands and Natural
Resource Operations, 2011). The population sites: (1) were located in the
ICH zone in the Columbia Basin, (2) were planted with lodgepole pine
between 1981 and 1996 (15– 30 years prior to field sampling), (3) had
no other species except lodgepole pine planted and (4) met provincial
standards for free growing prior to 2011.
Sampling from across the geographic range of the Columbia Basin was
stratified, with sites randomly selected in three site preparation categories
(no site preparation, broadcast burn and mechanical site preparation) and
three stand tending categories (no stand tending, pre-commercial thinning
and weeding). Broadcast burning is the controlled application of fire across
a cut block to reduce slash (logging debris such as branches) and/or vegetation density. Mechanical site preparation involves the use of heavy equipment to physically alter slash, forest floor and mineral soil layers by a variety
of methods, including scalping (exposing patches of mineral soil), disc trenching (creating continuous or intermittent furrows), mixing (chopping together vegetation, forest floor and mineral soil) and mounding (creating
raised planting spots). Pre-commercial thinning involves mechanically
cutting trees out of juvenile stands, usually with chainsaws, to reduce
tree density and competition between trees in a stand, usually with the
objective of concentrating growth on fewer high-quality trees. Weeding
involves removing or reducing abundance of vegetation or broadleaf trees
that are competing with crop trees. The weeded sites in our study were
treated by manual cutting rather than herbicide application.
We sought to randomly sample nine sites in each site preparation category and nine sites in each stand tending category. This was not possible,
however, due to insufficient numbers of accessible sites with the required
treatment history. Instead, seven burned, eight mechanically prepared
and 12 sites with no site preparation were randomly sampled (n ¼ 27).
For standing tending, seven of these were weeded, five pre-commercially
thinned and 15 were not treated (n ¼ 27). Although some sites had both
site preparation and stand tending treatments applied, these two treatment categories were evaluated as individual effects.
Field sampling
All field sampling was conducted in July and August, 2011. At each site,
six 50 m2 (r ¼ 3.99 m) plots were systematically established at 50-m intervals along a randomly located transect. Site series, elevation, slope gradient, aspect, slope position and absolute soil moisture regime (i.e. based
on a conversion from a relative to absolute scale for sites in the
347
Forestry
Table 2 Summary of site, climatic and tree species characteristics of the study sites by biogeoclimatic subzone
Variable description
Number of sites
Climatic region
Temperature class
Climate descriptiona
Mean annual temperature (8C)b
Mean warmest monthly
temperature (MWMT) (8C)b
Mean coldest monthly
temperature (MCMT) (8C)b
MAP (mm)b
MSP (mm)b
Precipitation as snow (PAS) (mm)b
Total number of frost-free days
(NFFD) (days)b
Number of continuous frost-free
days (FFP) (days)b
Elevation (m)
Slope (%)
Forest characteristicsa
Climax tree speciesa
Seral tree speciesa
Biogeoclimatic subzone
ICHmw
ICHmk
ICHdw
ICHdm
17
Moist
Warm
Hot moist summers
Very mild winters with light
snowfall
3.7
15.0
2
Moist
Cool
Warm wet summers
Cool winters with
moderate snowfall
3.6
15.1
3
Dry
Warm
Very hot moist summers
Very mild winters with
light snowfall
5.1
16.3
5
Dry
Mild
Hot dry summers
Cool winters with light
snowfall
3.7
15.4
27.7
27.8
25.9
28.1
914
330
374
163
841
333
334
159
747
264
260
180
860
277
386
161
92
86
108
89
680– 1492
0– 60
Moderate-to-recurrent fire
return periods have led to a
mosaic of seral and climax
stands.
1099 –1240
35 –60
Extensively disturbed by
wildfire so climax
stands are rare.
Seral stands
dominated by Pl are
common.
1053– 1590
5 –40
Mixed stands of Lw, Pl, Hw,
Cw, Sxw and Bl are most
common.
Broadleaf trees are
uncommon.
Hwc, Cw
Fd, Lw, Sxw, Bl, (Hw, Cw)
Cw, Sxw, Bl
Pl, Fd, Lw
913–1188
5 –50
Fire origin stands of Fd
and Lw very common.
Many stands originated
around 1900 from fires
set by mines.
Few climax or old
growth stands exist.
Cw, Hw
Fd, Lw, Pw, Ep
Cw, Hw (Sxw, Fd, Lw)
Cw, Hw, Fd, Bl, Lw, Pl
a
General subzone characteristics from Braumandl and Curran (1992) and Braumandl and Dykstra (2005).
Determined from Climate BC for the period 1971 –2000 (Wang et al., 2006) based on latitude and longitude recorded at individual sites.
c
Tree species codes (in alphabetical order): Bl (subalpine fir, Abies lasiocarpa), Cw (western redcedar, Thuja plicata), Ep (paper birch, Betula papyrifera), Fd
(Douglas-fir, Pseudotsuga menziesii), Hw (western hemlock, Tsuga heterophylla), Lw (western larch, Larix occidentalis), Pl (lodgepole pine, Pinus contorta)
and Sxw (hybrid spruce, Picea engelmannii x glauca). Less common species are in parentheses.
b
Kootenay– Boundary Forest Region) were recorded at each plot using
methods described by British Columbia Ministry of Forests (British Columbia
Ministry of Forests and Range and British Columbia Ministry of Environment,
2010). Aspect was recorded in degrees and later converted to the continuous variable, northness (cosine (aspect ×3.14159/180)). Soil moisture
regime was based on site characteristics (presence of indicator species in
the understory vegetation, slope position, slope gradient, aspect and soil
texture) and was converted to a categorical scale of 2 (very dry) to 6 (very
moist). Within each plot, total, well-spaced and free-growing conifer densities were recorded based on provincial standards (British Columbia Ministry
of Forests, 2000) and site-specific silviculture prescriptions. Densities of
broadleaf trees (trembling aspen, paper birch and black cottonwood)
were also recorded. Planted lodgepole pine were distinguished from naturally regenerated lodgepole pine based on their regular spacing and difference in size. All other tree species were counted as naturally regenerated.
All trees were assigned to a height class (,2, 2– 4 and .4 m) and diameter
at breast height (1.3 m from the base) (DBH) was recorded for all wellspaced trees of .1.3 m tall. Well-spaced trees are healthy trees of preferred
348
or acceptable species that are at least the minimum horizontal inter-tree
distance (usually 2 m) from other well-spaced trees. If they meet
minimum height requirements and are not impeded by vegetation competition, they are also free growing. Symptomatic presence of all disease,
insect, animal and abiotic damage were recorded for all trees according
to Henigman et al. (2001). Damage was classified as ‘unacceptable’
(failing to meet the health standards defined by the British Columbia provincial free-growing guidelines) or ‘acceptable’ (not serious enough to disqualify the tree from being classified as free growing). Standards for acceptable
and unacceptable damage are found in the ‘Establishment to Free Growing
Guidebook’ (British Columbia Ministry of Forests, 2000) and include the following: unacceptable galls caused by western gall rust (Endocronartium
harknessii (J.P. Moore) Y. Hirat) are those found on the main stem or on
branches within 5 cm of the main stem. Bear damage is unacceptable
when scars from bark peeling encompass greater than one-third of the
tree’s circumference. Criteria for unacceptable snow and ice damage
depend upon the type of damage (e.g. broken stems, forks, bends). Foliar
diseases are unacceptable when ≥80 per cent of the crown is affected.
Evidence against planting lodgepole pine monocultures
All instances of white pine blister rust are unacceptable and western white
pine was not a preferred or acceptable species on any of our study sites.
Broadleaf trees are also not considered crop species in the Columbia Basin.
conifer density but the variation among subzones was not as
great (range 1267–1743 stem ha21).
Free-growing and well-spaced stocking
Analysis
Summary statistics describing damage incidence (acceptable and unacceptable), total, well-spaced and free-growing densities, and height
and DBH of lodgepole pine and other species were produced. Logistic regression analysis and odds ratios were used to determine whether the incidence
of damage on lodgepole pine from the three most commonly observed
agents (western gall rust, snow and ice, and bears) were associated with climatic, location, silvicultural treatment, or site or stand factors. The general
form of the model was:
P(Y) = exp(b0 + b1 x1 + b2 x2 + . . . + bk xk )/(1 + exp(b0 + b1 x1
+ b2 x2 + . . . + bk xk ))
where P(Y) is the probability of incidence of a damaging agent. b0 is the
intercept, b1, . . ., bk are estimated coefficients and x1, . . ., xk are independent
climatic, location, silvicultural treatment, or site or stand variables. Three
regressions were used, one each for western gall rust, snow and ice, and
bears.
Logistic regression analysis was done using SAS PROC LOGISTIC (SAS Institute, Inc. 2002– 2003). A total of 162 plots were used in the analysis. The
events/trials syntax of PROC LOGISTIC was used where the events were
equal to the number of trees in a plot affected by a given agent, and trials
were equal to the total number of trees in the plot. Latitude, longitude, elevation, northness, slope gradient, stand and climatic variables were continuous predictive factors, soil moisture and slope position were
categorical and the silviculture factors were binary (coded by plot as ‘yes’
or ‘no’).
For each of the three common damaging agents, a stepwise regression
model was fitted with predictive factors allowed to enter or leave the model
automatically using relative probability Wald x 2 ≤ 0.05 as the default criteria for inclusion. The order in which individual factors were accepted
into the model reflects their relative probability x 2 value and therefore
their relative importance in the model. For each factor included in the
final model, odds of the agent occurring (i.e. the probability of an event
divided by the probability of non-event) were calculated and an odds
ratio (the multiplicative factor by which risk changed when the independent
variable increased by one unit) was determined. Due to the logarithmic
nature of odds ratios, a change in ‘x’ units of the predictive factor corresponded to a change in risk of the damaging agent equivalent to the
odds ratio raised to the power ‘x’. Odds ratios above 1 indicated increased
risk and those below 1 indicated decreased probability. The validity of the
final models was assessed using the Hosmer– Lemeshow (H-L) statistic
(Hosmer and Lemeshow 1980), which indicates the extent to which the
model provides better fit than a null model with no predictors. A nonsignificant H-L statistic (P ¼ 0.05) provides evidence the model adequately
fits the data.
Results
Total stocking
Total conifer stocking averaged 4629 stems ha21 across all subzones and tended to be highest in the ICHmw, then the ICHdm
and ICHdw and much lower in the ICHmk subzone, where most
of the trees on the sites were lodgepole pine (Table 3). Lodgepole
pine density (planted plus natural) averaged 1599 stems ha21
across all sites and trends by subzone followed that of total
All plantations had met minimum free-growing requirements prior
to 2011 (i.e. ≥700 free-growing stems ha21 as defined by the
British Columbia Ministry of Forests (2000)), but at age 15 –30
years, one-third had ,700 free-growing stems ha21 and very
few met the target free-growing density of 1200 stems ha21
(Table 4). Forty-eight per cent of sites were within 100 stems
ha21 of the minimum required free-growing density. Performance
was best where other tree species were abundant and total stocking was high. Free-growing stands averaged 3977 total stems ha21
and stands that were not free growing averaged 1141 total stems
ha21 (data not shown).
Species composition and diversity
Fifteen per cent of our stands were comprised of ≥80 per cent
lodgepole pine, and 19 per cent had free-growing stocking comprised of ≥80 per cent lodgepole pine (Figure 1). Natural regeneration of species other than lodgepole pine averaged 65 per cent
of the total stocking (Table 3) but varied considerably among
sites. The ICHmk subzone had the lowest density of non-lodgepole
pine trees (average 450 stems ha21 or 26 per cent of the total
stocking) and the ICHmw subzone the highest (average 3837
stems ha21 or 69 per cent of the total stocking). An average onequarter of the natural regeneration (1153 stems ha21) in the lodgepole pine plantations was western redcedar or western hemlock.
The other dominant conifer species were Douglas-fir, western
larch, western white pine, hybrid spruce and subalpine fir. Density
of broadleaf trees (black cottonwood, paper birch and trembling
aspen) was variable and averaged 175 stems ha21 across the 27
plantations. Excluding one ICHmw site and one ICHmk site that
each had .1000 broadleaf stems ha21 reduced the average
broadleaf density to ,50 stems ha21.
Planted lodgepole pine was larger in height and diameter than
natural pine or the other species (data not shown). Fifty-seven per
cent of lodgepole pine (including naturals) were in the tallest
(.4 m) height class, whereas only 16 per cent of the other
conifer species were .4 m tall and 59 per cent were ,2 m tall.
However, western larch, western white pine and to a lesser
degree Douglas-fir tended to be taller than the climax species.
Twenty-five per cent of broadleaf trees were .4 m tall and 26
per cent were ,2 m tall. The average DBH of well-spaced lodgepole
pine was 9.4 cm, which is greater than any of the other species
(Douglas-fir 7.0 cm; western larch, western red cedar and subalpine
fir 6.3 cm; western hemlock 5.1 cm and hybrid spruce 4.6 cm).
Damage
Overall, damage levels to lodgepole pine were high (66 per cent of
trees had damage and 44 per cent had unacceptable damage)
whereas non-lodgepole pine species tended to be healthier (39
per cent of trees had damage and 12 per cent had unacceptable
damage) (Table 5). Lodgepole pine suffered damage from a total
of 18 agents across all sites surveyed. Unacceptable damage
was the major factor contributing to reduced free-growing and
well-spaced stocking levels in the plantations. The three most
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Forestry
Table 3 Total, well-spaced (WS) and free-growing (FG) stocking of conifer tree species at the study sites averaged by biogeoclimatic subzone
Tree species
Stocking (stems ha21) and proportion of stocking comprised of each species
ICHmw (moist warm)
ICHmk (moist cool)
ICHdw (dry warm)
Total
All conifers
Stocking
5580
%
100
Lodgepole pine
Stocking
1743
%
31
Non-lodgepole pine
Stocking
3837
%
69
Western redcedar
Stocking
900
%
16
Western hemlock
Stocking
806
%
14
Douglas-fir
Stocking
815
%
15
Western larch
Stocking
180
%
3
Western white pine
Stocking
231
%
4
Hybrid spruce
Stocking
573
%
10
Subalpine fir
Stocking
329
%
6
Grand fir
Stocking
4
%
,1
All broadleaves
Stocking
434
%
100
Paper birch
Stocking
359
%
83
Trembling aspen
Stocking
71
%
16
Black cottonwood
Stocking
4
%
1
All sites
WS
FG
Total
WS
FG
Total
WS
FG
Total
WS
FG
Total
WS
FG
1086
100
962
100
1717
100
567
100
516
100
2845
100
844
100
712
100
3697
100
820
100
713
100
4629
100
972
100
855
100
439
40
414
43
1267
74
500
88
483
94
1289
45
400
47
389
55
1427
39
307
37
300
42
1599
35
415
43
395
46
647
60
548
57
450
26
67
12
33
6
1556
55
444
53
323
45
2200
61
513
63
413
58
3030
65
557
57
460
54
108
10
86
9
317
18
0
0
0
0
600
21
89
11
67
9
27
1
13
2
13
2
622
14
80
8
64
7
108
10
76
8
0
0
0
0
0
0
0
0
0
0
0
0
27
1
20
2
20
3
513
11
72
7
52
6
202
19
184
19
33
2
33
6
0
0
568
18
157
19
67
9
473
13
127
15
93
13
660
14
171
18
141
16
33
3
33
3
33
2
33
6
33
6
333
12
189
22
189
27
1281
35
273
33
247
35
390
8
95
10
90
11
0
0
0
0
0
0
0
0
0
0
22
0
0
0
0
0
0
0
0
0
0
0
148
3
0
0
0
0
137
13
120
12
0
0
0
0
0
0
0
0
0
0
0
0
240
7
60
7
33
5
405
9
97
10
82
10
55
5
47
5
67
4
0
0
0
0
11
1
0
0
0
0
167
5
20
2
7
1
244
5
38
4
31
4
4
,1
2
,1
0
0
0
0
0
0
22
1
11
1
0
0
7
,1
0
0
0
0
6
,1
4
,1
1
,1
350
100
56
100
20
100
175
100
0
0
56
100
0
0
99
57
100
29
0
0
7
35
53
30
250
71
0
0
13
65
23
13
common damaging agents on lodgepole pine were western gall
rust, snow and ice, and bears, and because of their prevalence
and severity, they are the focus of this paper.
Western gall rust occurred on 97 per cent of sites and affected
39 per cent of lodgepole pine on average (Table 5) (range 3 –89
350
ICHdm (dry mild)
per cent of lodgepole pine infected on individual sites). Seventy
per cent of the occurrences were serious enough to reject the
tree as free growing.
Snow and ice damage to lodgepole pine occurred on 82 per cent
of sites and affected an average of 7 per cent of the pine. The
Evidence against planting lodgepole pine monocultures
Table 4 Percentage of sites meeting minimum and target well-spaced and free-growing densities
Subzone No. of
sites
ICHmw
ICHdm
ICHdw
ICHmk
All
17
5
3
2
27
Sites meeting criteria (%)
Target
well-spaced
Minimum
well-spaced
.800 well-spaced
stems ha21a
Target
free-growing
Minimum
free-growing
.800 free-growing
stems ha21a
41.7
20.0
0
0
22.2
94.1
80.0
100.0
0
85.2
76.4
20.0
66.7
0
44.4
13.3
0
0
0
7.4
82.3
60.0
33.3
0
66.7
71.6
20.0
0
0
48.2
a
Sites with ,800 well-spaced or free-growing stems ha21 are within 100 stems ha21 of the minimum requirement.
Figure 1 Proportion of assessed plantations comprised of 0– 20, 21 –40,
41– 60, 61– 80 and 81–100 per cent of lodgepole pine according to (a)
total number of trees present and (b) number of free-growing trees.
maximum proportion of lodgepole pine trees damaged by snow
and ice was 74 per cent on an individual site, and 15 per cent of
sites had .25 per cent lodgepole pine damaged. Effects of snow
and ice included bends, basal sweep, leans, severely distorted
form, forks, crooks, broken stems and trees pushed right over.
That snow and ice were the cause of these defects was based on
visual evidence, local experience and historic snow loads. Broken
stems within the live crown and bends in the bole were prevalent
but mortality due to breakage below the live crown or uprooting
was uncommon. Overall, the percentage of trees with any type
of snow and ice damage was similar for lodgepole pine and most
other species, but with lodgepole pine, 60 per cent of the
damage was unacceptable, whereas the percentage of unacceptable incidences ranged from 0 per cent for western white pine to
28 –36 per cent for subalpine fir and hybrid spruce, and 43 –47
per cent for western red cedar, western hemlock and Douglas-fir.
Western larch was the only species with as much unacceptable
snow and ice damage as lodgepole pine (63 per cent of the
damage on western larch was unacceptable).
Bear damage, involving stripping bark off the lower portion of
tree trunks, occurred on 78 per cent of sites and affected an
average of 6 per cent of lodgepole pine. Bears damaged at least
15 per cent of lodgepole pine trees on one-third of the sites and
in one plantation 52 per cent of the lodgepole pine were affected.
Three-quarters of occurrences of bear damage were serious
enough to reject trees as free growing. Damage was most
common on larger diameter, dominant trees (10 –20 cm in diameter; data not shown) and almost exclusively on lodgepole pine, although 2 per cent of western white pine and 0.3 per cent of western
larch were also affected. No damage was seen on Douglas-fir,
western redcedar, western hemlock, hybrid spruce, subalpine fir
or broadleaf trees. Damage tended to be clustered, even in relatively
uniform plantations.
Other types of damage were less prevalent than western gall
rust, snow and ice, or bears, but important on some sites. Atropellis
canker (Atropellis piniphila (Weir) (Lohman & Cash)) infected up to
19 per cent of pine on an individual site, and all incidences caused
trees to be unacceptable as crop trees. Lodgepole pine terminal
weevil (Pissodes terminalis Hopping) and sequoia pitch moth
(Synanthedon sequoia Hy. Edw.) were the major insect pests, with
up to 21 –27 per cent of trees affected on a single site, respectively.
However, each type of insect damage was present on only about
10 per cent of sites. Twenty-two per cent of terminal weevil
damage was unacceptable; sequoia pitch moth damage was
never considered unacceptable. The incidence of pine terminal
weevil damage is probably higher than we report because the
reason for many forks and crooks could not be determined. Lodgepole pine suffered foliar diseases at about one-third of the plantations but damage was never unacceptable. In almost all affected
plantations, ,10 per cent of trees were diseased, and on most
trees, ,5 per cent of needles were affected. Root disease symptoms were seldom seen, except on one site where 23 per cent of
pine suffered from Armillaria root disease (Armillaria solidipes
Peck). Ungulate browsing affected 16 per cent of lodgepole pine
in one plantation, but dominant (crop) trees were too tall for their
leaders to be recently browsed, and few trees of any size had
extensive clipping of laterals. Damage from silviculture treatments
was present in 10 per cent of plantations and was rare on canopy
layer trees. In the most extreme case, 26 per cent of the understorey
trees were damaged by pre-commercial thinning and pruning.
Some trees that were not targeted for removal were partially cut,
and others were damaged by falling trees and branches, resulting
in wounds, crooks, forks and severe bends. Damage from conifer
and broadleaf tree competition occurred on 5 per cent of all
trees and was less prevalent on lodgepole pine than other species.
The main effects were abrasion, forks, crooks and bends.
The major damaging agents on the natural regeneration were
white pine blister rust (Cronartium ribicola J.C. Fisch.) and foliar diseases on western larch, including larch needle blight (Hypodermella laricis (Tub.)). Seventy-three per cent of western white pine
351
Forestry
Table 5 List of all damaging agents recorded on lodgepole pine and other species and their frequency of occurrence
Damaging agent
Diseases
Western gall rust
Atropellis canker
White pine blister rust
Armillaria root
disease
Foliage diseases
Insects
Lodgepole pine
terminal weevil
Spruce terminal
weevil
Sequoia pitch moth
Cooley spruce gall
adelgid
Animal damage
Bear
Moose and deer
Hare
Squirrel
Unknown
Abiotic
Snow and ice
Frost
Drought
Vegetation
Trees (including
broadleaves)
Shrubs
Treatment damage
Pre-commercial
thinning
Other
Falling trees/rolling
logs
Lodgepole pine
All other species
Proportion of
sites
Proportion of
trees damaged
Proportion of trees with
unacceptable damage
Proportion of
sites
Proportion of
trees damaged
Proportion of trees with
unacceptable damage
97.3
37.0
0.0
11.1
39.0
1.3
0.0
0.7
31.7
1.3
0.0
0.7
0.0
0.0
37.0
11.1
0.0
0.0
2.6
0.2
0.0
0.0
2.6
0.2
29.6
2.2
0.0
55.6
8.3
0.2
11.1
0.7
0.2
0.0
0.0
0.0
0.0
0.0
0.0
7.4
0.0
0.0
7.4
0.0
0.7
0.0
0.0
0.0
0.0
55.6
0.0
7.0
0.0
0.0
77.8
48.1
11.1
3.7
29.6
5.9
4.2
0.2
0.1
0.8
4.4
1.8
0.1
0.0
0.1
11.1
18.5
18.5
0.0
11.1
0.4
0.9
0.4
0.0
0.4
0.1
0.3
0.2
0.0
0.2
81.5
3.7
3.7
7.4
0.1
0.1
4.5
0.1
0.0
55.6
3.7
7.4
7.7
0.1
0.1
3.3
0.0
0.1
33.3
4.3
2.7
70.4
4.6
1.5
25.9
0.8
0.5
33.3
0.9
0.2
11.1
0.8
0.2
14.8
2.8
1.2
33.3
0.8
0.2
44.4
1.0
0.7
trees were infected by blister rust (not including trees already
dead). However, white pine averaged only 3 per cent of total stocking, was never a major stand component and occurred only in the
ICHmw subzone. Foliar diseases affected two-thirds of the western
larch, and on average, 21 per cent of needles were dead on the
affected trees, but only 2 per cent of larch had unacceptable
damage.
Predictors of damage
All three logistic regression models appear to fit the data as evidenced by P-values greater than a ¼ 0.05 in the Hosmer and Lemeshow Goodness-of-Fit test (Table 6). Our logistic regression model
predicted increased risk of western gall infection with increased soil
moisture, more northerly aspects and mechanical site preparation
352
Table 6 Results of Hosmer and Lemeshow goodness-of-fit test for the
three logistic regression models
Damaging agent
Western gall rust
Snow and ice
Bear damage
Hosmer and Lemeshow goodness-of-fit test
x2
DF
Prob. . x 2
11.19
8.82
14.06
8
8
8
0.19
0.36
0.08
(Table 7). Pre-commercially thinned stands were associated with
lower risk of the disease than un-thinned ones. Our model predicted an increase in snow and ice damage risk moving from
Evidence against planting lodgepole pine monocultures
Table 7 Summary of odds ratios (logarithmic scale) and probability of .x 2 for the mean, for factors predicting the presence of damaging agents
Factor types
Factor
Units
Western gall rust
Odds ratio
Climatic
Location
Site
Stand
Treatment
MWMT
MCMT
TD
MAP
AH:M
SH:M
DD , 0
DD . 5
NFFD
PAS
Elevation
Latitude
Longitude
Soil moisture
Slope gradient
Slope position
Northness
Stand age
Pine density
Broadcast burn
Mechanical site preparation
Pre-commercial thinning
Weeding
18C
18C
18C
1 mm
1 unit
1 unit
1 degree-day
1 degree-day
1 day
1 mm
1m
18
18
1 unit
1%
lower vs mid
1 unit
1 year
200 stems ha21
Yes vs No
Yes vs No
Yes vs No
Yes vs No
P . x2
1.81
,0.0001
9.48
,0.0001
Snow and ice
Odds ratio
P . x2
1.01
,0.0001
0.51
0.0012
2.32
2.01
0.46
,0.0001
0.0132
Bear damage
0.0012
Odds ratio
P . x2
2.32
0.0019
0.83
2.43
0.0019
,0.0001
3.63
0.0200
Bold values indicate an odds ratio of .1, thus predicting an increase in the damaging agent with an increase in the climatic, location or site factor or with the
application of a silviculture treatment. Bold italic values indicate an odds ratio of ,1, thus predicting a decrease in the damaging agent with an increase in
the factor. White cells indicate no effect as predicted by logistic regression. An increase of ‘x’ units of the predictor factor corresponds with a change in risk of
the damaging agent equivalent to the odds ratio raised to the power of ‘x’.
MWMT, mean warmest monthly temperature; MCMT, mean coldest monthly temperature; TD, continentality; MAP, mean annual precipitation; AH: M,
annual heat: moisture index; SH: M, summer heat: moisture index; DD , 0, no. of degree-days ,08C; DD . 5, no. of degree-days .58C; NFFD, no. of frost-free
days; PAS, precipitation as snow.
west to east. Risk of snow and ice damage was also associated with
increasing MAP and broadcast burning. Our model indicated a
strong association between bear damage and pre-commercial
thinning treatments as well as declining pine density. An increased
risk of bear damage was also associated with increased soil moisture and broadcast burning.
Discussion
Six questions were posed at the onset of this study. The first was
whether lodgepole pine plantations that were declared free
growing are still meeting this criteria at age 15–30 years. We
found that 30 per cent of them now fail as free growing, but this
failure rate is much lower than the 70 per cent rate found by
Mather et al., (2010) in a study using the same methods as ours,
which was conducted in other parts of southern interior British Columbia. In the northern interior of the Province, 18 per cent of stands
(not limited to ICH lodgepole pine plantations) that had been
declared free growing no longer met this requirement when they
were re-surveyed 5 years later (Woods and Bergerud, 2008). In
our study and Mather et al. (2010), about half of the plantations
had free-growing and well-spaced stocking levels of ≤100 stems
ha21 above the minimum requirement of 700 stems ha21. The
Forest Practices Board (2003) considered stands in this category
to be at high risk of not achieving their full productivity potential,
which is of particular significance in the ICH zone because of its
high growth potential and contribution to timber supply in British
Columbia.
Our second question addressed stocking and species composition of natural regeneration in the lodgepole pine plantations
and the degree to which it is contributing to stocking. The
amount and species composition of natural regeneration was
highly variable among sites, but overall it made a substantial contribution to stocking. In our study, free-growing performance was
best where other tree species were abundant, emphasizing the
benefits of natural seeding to forest regeneration. Lower freegrowing success in Mather et al. (2010) may in part have been
due to lower densities of natural regeneration (1748 total
non-pine stems ha21 vs 3030 in our study). A diversity of naturally
regenerating species prevented many of our stands from being
monocultures (i.e. ≥80 per cent of stocking comprised of one
353
Forestry
species), but we found a much higher proportion of monoculture
stands than the overall ICH average reported for the southern interior of British Columbia. The provincial government reports that only
2 per cent of free-growing southern interior ICH stands are lodgepole pine monocultures and that lodgepole pine monocultures
are decreasing with harvesting (from 5 per cent pre-harvest to 2
per cent post-harvest) (British Columbia Ministry of Forests and
Range, 2008). However, Stevenson et al. (2011) report that the
abundance of stands dominated by lodgepole pine in the ICH
zone has increased over time, from 4 per cent of stands selected
for harvest to 10 per cent of regenerated stands (note that such
comparisons should be interpreted cautiously due to differences
in sampling strategies and populations). An increasing trend
towards single-species stands is likely to continue because the
area of monocultures (all species and all biogeoclimatic zones
combined) at the point of free-growing assessment has increased
by 9 per cent since licensees and British Columbia Timber Sales
assumed the primary silvicultural obligation for reforestation in
1987 (British Columbia Ministry of Forests and Range, 2008).
An average one-quarter of the natural regeneration in our
lodgepole pine plantations was western redcedar or western
hemlock and although the trend has been to reforest with seral
species such as lodgepole pine, western redcedar has recently
been gaining acceptance as a planted species in the ICH zone
(Stevenson et al., 2011). Thus, climax species are still maintaining
a presence in second growth ICH stands; however, their early
growth is slower than that of the seral species. This resulted in an
average of only 13 per cent (116 stems ha21) of the free-growing
stocking being comprised of western redcedar and western
hemlock. During pre-commercial thinning operations, even the
largest cedar and hemlock trees are frequently removed, so the
presence of these two species post-thinning is often limited to
very small trees that escape cutting and may not reach merchantable size when the seral species mature.
The third question we asked regarded the type, extent and
incidence of damage in 15- to 30-year-old lodgepole pine plantations. Damage levels were high on lodgepole pine whereas natural
regeneration, with the exception of western white pine, tended to
be healthier. The prevalent foliar diseases on larch are generally
associated with only minor growth reductions (Henigman et al.,
2001). Total damage levels were comparable with those found by
Heineman et al. (2010) in the ICH zone elsewhere in southern interior British Columbia, but they found a higher proportion of pine with
unacceptable damage (61 per cent vs 44 per cent). Vyse et al.
(2013) found that mountain pine beetle was the most important
damaging agent in 20- to 26- year-old ICH lodgepole pine-planting
trials. Their study was based on a small sample of plantations not
located in the Columbia Basin. Regardless, the high levels of
damage found in our study and these others show that singlespecies planting of lodgepole pine in the ICH zone is a questionable
regeneration strategy unless considerable losses are accepted and
planned for. Damaged planted lodgepole pine takes up valuable
growing space in young stands and expected quality reductions,
as well as growth and mortality losses, suggest that other species
should be used to a greater degree.
That we found very high levels of western gall rust incidence is
not surprising since others have found similar results elsewhere,
and western gall rust is the most common stem rust of lodgepole
pine in Canada (van der Kamp and Tait, 1990). Twenty-seven per
cent of lodgepole pine were affected in the ICH zone across the
354
southern interior of British Columbia (Mather et al., 2010), 49 per
cent in northern British Columbia (van der Kamp et al., 1995),
.20 per cent of trees on one-third of sites in northern interior
British Columbia (Woods and Bergerud, 2008), 43 per cent in westcentral Alberta (Blenis and Duncan, 1997) and 67 per cent in trials
of 53 provenances from British Columbia, Alberta and the Yukon
(Wu et al., 1996). As with our results, infection rates varied considerably among sites and exceeded 85 per cent of trees on some
sites. Western gall rust has long been recognized to cause significant losses in young lodgepole pine stands (Ziller, 1974; Hiratsuka
and Powell, 1976). Both stem and near-stem galls result in similar
outcomes for the tree (Wolken, 2008). They can result in growth
losses, poor tree form, reduced wood quality, restricted endproduct use and stem breakage and can ultimately cause mortality
or prevent trees from reaching merchantable size (Gross, 1983;
Blenis et al., 1988). These are much more serious than more
distal branch galls, which have little impact on the host tree, although they are an important source of spores that spread the
disease and, if very abundant, may reduce photosynthesis
(Gross, 1983). The magnitude of losses due to western gall rust is
not well documented, but reductions in volume of 7 per cent by rotation age have been estimated for central British Columbia
(Woods et al., 2000). A higher volume loss (15 per cent over a
20-year period) was reported for stands in Alberta (Bella and
Navratil, 1988). In one of the few published studies of mortality
rates due to western gall rust, Wolken (2008) projected that 57 –
62 per cent of stem-galled trees would not survive to age 80,
which translates to 15 per cent of the lodgepole pine on our
sites. However, mortality depends on the degree of gall encirclement of the stem and is highest when galls encircle .79 per cent
of the stem (Wolken, 2008).
We found that snow and ice was an important damage cause in
many juvenile Columbia Basin lodgepole pine plantations. Other
published data on snow and ice damage to lodgepole pine from interior British Columbia are limited. However, Heineman et al. (2010)
reported high levels of snow and ice damage in some lodgepole
pine plantations, but these were mainly .1600 m in elevation
(ESSF zone). Broken stems within the live crown, which we frequently observed, result in loss of dominance and a reduction in growth
rate. Although trees may recover satisfactorily, timber loss still
results due to crooks, forks or multiple leaders. Bends or sweep in
the lower part of the stem can be caused not only from snow accumulations on the tree, but also from lateral movement of the snowpack and result in reduced height growth and sometimes
compression failure on the concave or downhill side of the stem.
An indirect effect of snow damage is increased susceptibility to
fungal or insect attacks such as root rot and pine weevils
(Nykanen et al., 1997). In our study, lodgepole pine stood out as
having the most severe snow and ice damage of all species
except western larch, indicating low suitability of lodgepole pine
for regeneration in areas where high levels of snow damage have
previously been observed. European studies indicate that spruce
and subalpine fir withstand snow damage better than pine. This
is probably because they have a narrower crown, creating less
surface area for snow accumulation and concentrating snow
closer to the stem, thereby keeping the stem more stable
(Nykanen et al., 1997). It is unknown whether snow and ice
damage on our sites occurred during single storms or prolonged
snow loading, but stresses and defects result from both situations.
In Europe, extensive snow and ice damage to thousands of
Evidence against planting lodgepole pine monocultures
hectares of forest has sometimes occurred (Nykanen et al., 1997),
but this has not been the case in the Columbia Basin.
Bears were an important damaging agent, affecting 78 per cent
of our sites, in congruence with reports elsewhere. A single bear is
capable of stripping bark from 50 to 70 trees per day (Schmidt and
Gourley, 1992)or destroying 10 to 15 per cent of a forest stand each
year (Ziegltrum, 1994). In some areas, injury levels have affected
up to 87 per cent of trees (Sullivan, 1993), which exceeds what
we observed. Heineman et al. (2010) observed infrequent bear
damage in juvenile lodgepole pine plantations across southern interior British Columbia, although it occurred more often in the ICH
zone than other areas. Damage from bears can be detrimental to
the health and economic value of a stand because complete girdling kills the trees and partial girdling leads to a reduction in tree
growth rate during the recovery period, especially if wounds encompass .50 per cent of a tree’s circumference (Nelson, 1989).
The wounds also become entry points for decay fungi. Timber
yield may be seriously affected by bear damage, with estimates
of a 13 –17 per cent reduction in yield at rotation age (Brodie
et al., 1979; Mason and Adams, 1989), although Lowell et al.
(2010) reported a smaller impact (6 per cent lower log cubic
volume recovery). Lumber from bear-wounded trees is sometimes
degraded due to distortion, borer tunnels and decay (Childs and
Worthington, 1955). Lowell et al. (2010) reported that beardamaged trees yielded 23 per cent of the highest grade of
lumber compared with 35 per cent for undamaged trees. The
impacts of bear damage are made worse by the tendency for
animals to select the most vigorous trees, especially in thinned
stands where each tree comprises more of total stand volume.
Species other than lodgepole pine were probably undamaged on
our sites due to denser branches on the lower bole, which bears
avoid (Maser, 1967). In other locations in western North America,
lodgepole pine has also been a preferred species (Mason and
Adams, 1989; Barnes and Engeman, 1995). Our finding that dominant, vigorous trees were preferentially attacked has been
reported elsewhere (Maser, 1967; Fersterer, 2000). Most affected
trees were 10– 20 cm in diameter, agreeing with Mason and
Adams (1989). Juvenile trees (age 10 –45 years) are usually preferentially attacked, based on seven studies in the northwestern US
(Fersterer, 2000), indicating that our stands were of a highly susceptible age. The patchy nature of damage that we observed in
our stands agrees with observations from other areas (Childs and
Worthington, 1955; Schmidt and Gourley, 1992).
Our fourth question was whether climatic, site or stand factors,
or silviculture treatments are associated with increased risk of
damage to lodgepole pine from western gall rust, snow and ice,
or bears. Our model predicted increased risk of western gall rust
damage on moister sites and northerly aspects which supports
other studies showing that moister, cooler sites favour production,
release, germination and infection by western gall rust fungi
(Chang and Blenis, 1986; Chang et al., 1989; Adams, 1997). Risk
of damage from western gall rust was greater in unthinned than
thinned stands perhaps because stand criteria for thinning
included low infection rates and/or damaged trees were preferentially removed. However, other studies have found higher western
gall rust incidence in more open stands than denser ones (Hills
et al., 1994). Pre-commercial thinning could favour rust incidence
by changing microclimate, crown structure and inoculum levels,
which may affect spore dispersal and change the rate of infection
(Bella, 1985; van der Kamp and Spence, 1987). The larger, more
vigorous trees in thinned stands may be more favourable hosts
than smaller trees due to more surface area for spore deposition
(van der Kamp and Spence, 1987). Mechanical site preparation
was also associated with increased risk of western gall rust but
we are unable to explain this result. Although we found that risk
of western gall rust was associated with certain factors, van der
Kamp (1994) concluded that it is not possible to predict at the
time of stand establishment which lodgepole pine stands will
suffer significant damage from the disease. He emphasized
instead the concept of ‘wave years’ when conditions are just
right for infection, which may occur locally only once every 10
years. Some stands may never experience a wave year while in a
susceptible state, whereas others may experience several of
these years. The logistic regression model predicted an increase
in snow and ice damage risk moving from west to east, which
may be related to higher precipitation levels in the east caused
by orographic lift over the mountain ranges. That the model predicted a greater risk of snow and ice damage with increased MAP
is likely due to greater snow loads where the amount of winter precipitation is higher. The prediction of higher risk of snow and ice
damage on burned vs unburned sites may be explained by the
fact that burning reduces slash density, which could increase
snow movement. Higher risk of bear damage was associated
with increased soil moisture, which aligns with the preference of
bears for moist areas where plants are more succulent and nutritious (Herrero, 1985). Fersterer (2000) also reported that bears peel
more trees on moist sites than dry ones. Our model indicated a
strong association between bear damage and pre-commercial thinning treatments as well as declining pine density, which agrees with
others who have found that bears cause more damage in open
stands. Mason and Adams (1989) found that bear damage was
5–7 times higher in thinned than untreated stands, and Childs
and Worthington (1955) found it was 3–4 times higher in understocked compared with well-stocked stands. Bears prefer tree canopies sparse enough to let sunlight reach the forest floor (Van
Tighem, 1999), accessibility to tree boles is better in open stands
and thinning results in increased carbohydrate levels (Kimball
et al., 1998). Bear damage was associated with broadcast burning
possibly because of improved access following slash reduction.
The fifth question we posed was how management practices
can be adjusted to reduce the risk of damage from western gall
rust, snow and ice, and bears. Our results suggest management
practices favour mixed stands rather than monocultures of lodgepole pine. Silviculture treatments and harvesting operations are logistically simpler when management is done for single-species
stands, but plantation health may be reduced in stands dominated
by one species because most forest pests are species-specific and
tree diversity spreads the risk of damage (Woods et al., 2000; Jactel
et al., 2009;Gamfeldt et al., 2013). The recent mountain pine beetle
and Dothistroma needle blight outbreaks in British Columbia are
examples where catastrophic losses occurred in pine-dominated
stands and such epidemics are expected to increase with climate
change (Sturrock et al., 2011). Given that virtually all ICH sites in
the Columbia Basin are ecologically suited to species besides
lodgepole pine (British Columbia Ministry of Forests, 2000), it
would not be difficult to scale back planting of pine in favour of
more complex mixtures emulating natural stand composition.
This can be achieved by planting a variety of species and/or implementing silviculture systems that favour diverse natural regeneration. Both pest and long-term productivity concerns with
355
Forestry
lodgepole pine have been identified by others as reasons to curtail
the use of lodgepole pine (Vyse et al., 2013)and the Chief Forester of
British Columbia recently made recommendations that support a
reduction in the prolific use of this species (Snetsinger, 2011).
Results from ours and other studies suggest that pre-commercial
thinning that favours lodgepole pine should be avoided in regions
where bear damage levels are historically high, especially on moist
sites. Our results indicate that up to half of the lodgepole stems in
thinned plantations can be damaged by bears. Unlike other
studies, we provide no evidence of increased risk of western gall
rust in thinned stands. Broadcast burning was associated with
increased risk of snow and ice and bear damage, and mechanical
site preparation was associated with increased risk of western gall
rust. However, the choice to utilize these site preparation treatments
must consider many other factors such as their impact on plantable
spots as well as natural regeneration.
Our final question was whether damage to lodgepole pine will
increase given climate change predictions. Our study took place
across a relatively small geographic area, explaining why we
found few associations between risk of lodgepole pine damage
and climatic factors. However, climate change has already contributed to two recent major insect and disease epidemics in lodgepole
pine forests in British Columbia: the mountain pine beetle and
Dothistroma needle blight (Woods et al., 2010). The warming
climate predicted for the Columbia Basin in particular is expected
to result in increased insect outbreaks, through increased diversity
and activity of insects and through increased drought stress and
weather-related damage of forests. Further damage to lodgepole
pine plantations by sequoia pitch moth, lodgepole pine terminal
weevil and mountain pine beetle can be expected. Snow damage
may become more common because of the likelihood of more
snow events at temperatures near freezing, which is when snow
best adheres to tree crowns (Nykanen et al., 1997). The possibility
of increased use of trees for food by bears (i.e. increased damage
due to stripping of bark) exists if food acquisition becomes more
difficult due to either drought in the feeding season or earlier emergence from hibernation, when plants are still undeveloped. The
wetter climate predicted for the Columbia Basin, particularly
when combined with warmer temperatures, should favour pathogen populations as well, with increasing frequency of wave years
for stem rusts such as western gall rust, and increasing risk of
foliar diseases such as Dothistroma needle blight (Sturrock et al.,
2011). Summer drought stress will place lodgepole pine at more
risk of Armillaria root disease, Atropellis canker and lodgepole
pine dwarf mistletoe (Arceuthobium americanum Nutt. Ex Englemann) as well as alien invasive pests (Sturrock et al., 2011). The
compromised condition of lodgepole pine plantations in the Columbia Basin today places them at greater risk for future insect
and pathogen damage than healthier plantations that have been
regenerated to a more locally suitable and diverse mix of tree
species. Pest complexes, where multiple insects and diseases
co-exist in a single plantation, have the potential to build in these
lodgepole pine plantations, potentially leading to decline of planted
lodgepole pine forests in this area in the future.
Conclusions and recommendations
Single-species planting of lodgepole pine is common in the ICH
zone in southern interior British Columbia, which is the most
356
productive and tree-species diverse biogeoclimatic zone in this
region. Our results suggest that the majority of ICH lodgepole
pine plantations in British Columbia’s Columbia Basin will suffer
reduced potential stand productivity because damaged pine is
taking up valuable growing space that could be occupied by
other species with fewer problems. Stresses imposed by changes
in climate towards conditions that lodgepole pine is not adapted
to, or moving lodgepole pine away from its native range, are
likely to increase plantation losses. Our analysis found few associations between damage to pine and climatic conditions, but the
study area was fairly small. Site conditions, such as increasing
moisture, were associated with increased western gall rust and
bear damage. We found associations between silviculture treatments and damage. In particular, pre-commercial thinning can
increase incidence of damage to lodgepole pine from bears, and
broadcast burning was associated with increased snow and ice
damage and bear damage. In the ICH zone, we found that
natural regeneration was diverse and plentiful on many sites and
partially compensated for poor lodgepole pine performance.
Based on our findings, we recommend that single-species planting
of lodgepole pine in the ICH zone be curtailed and that, where possible, pre-commercial thinning operations in existing lodgepole
pine stands favour a diversity of species. Mitigating damage as
climate changes in the Columbia Basin should involve a combination of natural and artificial reforestation with a rich mix of
locally adapted and migrated tree species that together comprise
a resilient and stress-tolerant forest. Management of these stands
should involve monitoring, modelling, risk assessments and planning following a complex adaptive systems approach (Woods
et al., 2010; Sturrock et al., 2011; Filotas et al., 2013).
Acknowledgements
We gratefully acknowledge the support of Columbia Basin Trust (CBT), which
delivers social, economic and environmental benefits to the residents of the
Columbia Basin. We also thank the partners in the project: the University of
British Columbia, FORREX, the British Columbia Ministry of Natural Resource
Operations (MNRO), and the West Kootenay Ecosociety. Valuable assistance
during the initiation and implementation of the project were provided by
Deb McKillop and Dr. Lorraine Maclauchlan. Dr. Don Sachs conducted the
data analysis, and Robyn Simard and Mandy Ross assisted with field data
collection. We are very grateful for valuable input from the editor, Dr.
Helen McKay, and three anonymous reviewers.
Conflict of interest statement
None declared.
Funding
This work was supported by Columbia Basin Trust (CBT), who provided
funding under the Large Grant Application of the CBT′s Environmental Initiatives Program; the British Columbia Ministry of Forests, Lands and Natural
Resource Operations; and a Natural Science and Engineering Research
Council (NSERC) Discovery Grant for S.W.S.
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